1. Field of the Invention
The present invention relates to a wire bonding method and apparatus and more particularly to a wire bonding method and apparatus which detects the heights or levels of bonding surfaces.
2. Prior Art
Conventional examples of wire bonding apparatuses which are capable of detecting the level of the bonding surface are disclosed in Japanese Patent Application Publication ("Kokoku") Nos. 64-9730 and 1-31695.
In these apparatuses, a tool arm which holds a capillary is attached to an arm holder, and the arm holder is swingably attached to a lifter arm. The lifter arm is mounted to a bonding head so that the lifter arm is able to move up and down or is above to swing. In addition, a gap detecting sensor is attached to the lifter arm so that the sensor faces the tool arm.
When the lifter arm is lowered, or when the lifter arm is caused to swing so that the capillary side of the lifter arm is lowered, the capillary is moved down, and the gap between the tool arm and gap detecting sensor changes. The point in time where the value of the detection output derived from the gap change has changed by a given amount from the initial value is considered as the time of contact between the capillary and the bonding surface.
In this prior art, the level or the height of the bonding surface is ascertained according to the point in time where the value of the detection output has changed by a given amount from the initial value. In other words, the level is ascertained according to a given amount of change in the absolute value of the detection output.
If vibrations occur in the lifter arm and arm holder (especially when very small vibrations occur immediately prior to the contact of the capillary with the bonding surface), and also if noise enters the gap detecting sensor, etc., the level of the bonding surface is ascertained at a point other than the correct bonding surface. As a result, detection errors occur.
Furthermore, since the gap detecting sensor is a magnetic sensor, there is a response lag relative to the threshold value of the sensor. As a result, it takes time for the magnetic sensor to detect the bonding surface after the capillary has actually contacted the bonding surface. During this time, the capillary presses the bonding wire against the bonding surface, so that the amount of crushing of the bonding wire or ball, i.e., the so-called "sink-in", becomes excessive.